1. Field of the Invention
The present invention relates to a grinding machine and a grinding method for grinding a workpiece with a grinding wheel while coolant is being supplied to a contact surface on a grinding surface of the grinding wheel and the workpiece. Particularly, it relates to a grinding machine and a grinding method selectively using first and second grinding wheels in dependence upon the steps of grinding operations.
2. Discussion of the Related Art
Heretofore, in grinding a workpiece with a grinding wheel provided on a grinding machine, grinding burn, thermal stress and the like of the workpiece caused by the grinding heat are prevented by supplying coolant toward a grinding point between the workpiece and the grinding wheel for cooling and lubrication. However, where coolant is supplied toward the grinding point between the workpiece and the grinding wheel, a dynamic pressure is generated in the coolant between the workpiece and the grinding wheel. In particular, where the workpiece has a hole or groove, the same causes the dynamic pressure to fluctuate, which gives rises to a problem that the machining accuracy of the workpiece is deteriorated due to a relative displacement between the workpiece and grinding wheel. Japanese Utility Model Application No. 57-157458 discloses a technology for preventing the machining accuracy from being deteriorated due to such a dynamic pressure generated in the coolant.
In the technology described in the Japanese application, there is provided a coolant supply device capable of switching into two high and low levels the pressure of coolant supplied to a coolant nozzle which supplies coolant toward a grinding point at which the grinding wheel contacts a workpiece. The coolant pressure is switched into a high pressure during a rough grinding wherein the feed rate of the grinding wheel toward the workpiece is high, but into a low pressure during a finish grinding wherein the feed rate is low, as well as during a spark-out grinding. Thus, the machining accuracy is prevented from being deteriorated due to the dynamic pressure generated in the coolant.
However, in the prior art described above, it is impossible to release the dynamic pressure which is generated in the coolant supplied to a contact surface on which the grinding surface of the grinding wheel contacts the workpiece. In particular, where the rotational speeds of the grinding wheel and the workpiece are heightened to increase the grinding efficiency, the dynamic pressure generated in the coolant causes the machining accuracy to be deteriorated. For desired machining accuracy, it has to be done to lower the rotational speeds of the grinding wheel and the workpiece. This gives rises to a problem that the machining efficiency is lowered.